We demonstrated the binding of an IgM monoclonal protein, obtained from a patient with motor neuron disease, with known antibody activity against gangliosides GM1, GD1b, and asialo GM1, to neuromuscular junctions in guinea pig gastrocnemius muscle, using an indirect immunofluorescence technique. Staining disappeared after delipidation of muscle sections. Denervated muscle sections showed no labeling at the neuromuscular junction after incubation with the patient's serum. This indicates presynaptic binding of the IgM M protein and supports the concept that IgM monoclonal antibody to nerve terminal determinants may underlie a motor neuron disorder.